Temporal Variation in Immune Components of the White Grub Phyllophaga polyphylla (Bates) (Coleoptera: Melolonthidae)

Ecological immunology assumes that the host immune efficiency is correlated with its survival after pathogen challenge. To test this hypothesis, we challenged Phyllophaga polyphylla (Bates) larvae with the naturally occurring fungus Metarhizium pingshaense on two consecutive years (2011 and 2012). In each year, we injected the blastospores of M. pingshaense and then used levels of prophenoloxidase (proPO), phenoloxidase (PO) and total haemolymph serum protein as indicators of immune efficiency. Larvae were injected with (1) phosphate buffered saline (PBS) + Tween and viable blastospores of M. pingshaense, (2) PBS + Tween and non-viable blastospores of M. pingshaense, (3) PBS + Tween, or (4) non-manipulated. Overall, levels of PO, proPO and total haemolymph serum protein in larvae after 12 h were similar amongst treatments within each year of collection. However, larvae collected in 2011 showed higher PO and proPO activity but lower total haemolymph serum protein compared with larvae collected in 2012. A survival study injecting viable blastospores showed that larvae collected in both years died within 48 h; however, when non-viable blastospores were injected, which were still toxic to larvae, mortality was greater in larvae collected in 2011 compared with larvae collected in 2012. Altogether, these results indicate that PO, proPO and total haemolymph serum protein do not predict immune strength of P. polyphylla against blastospores of M. pingshaense, but higher values of PO and proPO were correlated with higher survival rates against non-infective but toxic agents. The possible role of some abiotic factors over the differences observed for immune components of P. polyphylla in different years of collection is discussed.